It is generally known that cyclopentene can give an unsaturated linear ring-opening polymer by metathesis ring opening polymerization in the presence of a so-called Ziegler-Natta catalyst consisting of a compound of a transition metal belonging to Group 6 in the Periodic Table like WCl6 and MoCl5 and an organometallic activating agent like triisobutyl aluminum, diethyl aluminum chloride, and tetrabutyl tin.
Conventionally, as the ring-opening polymer of cyclopentene obtained by the process described above, a ring-opening polymer of cyclopentene having a high trans ratio is used from the view point of having crystallinity and excellent green strength. However, although the ring-opening polymer of cyclopentene having a high trans ratio has excellent green strength and wear resistance, due to high crystallinity, it has a poor rubber property at low temperature, and therefore there has been a problem that it cannot be used for various rubbers that are used at low temperature, for example, a rubber material for a tire.
In this connection, a method of lowering crystallinity and improving the rubber property at low temperature of the ring-opening polymer of cyclopentene by increasing the cis ratio of the ring-opening polymer of cyclopentene has been studied. For example, according to Non-Patent Document 1, a ring-opening polymer of cyclopentene having a high cis ratio, non-crystallinity, and glass transition temperature of −100° C. or less is obtained by using MoCl5/triethyl aluminum or WCl6/trialkyl aluminum as a polymerization catalyst.
Meanwhile, it is recently required for a rubber material for a tire, for example, to have excellent cloud resistance, i.e., low heat generation property, to satisfy the low fuel requirements for an automobile. As a method for improving the low heat generation property of a rubber material, a method of adding a filler like silica or carbon black to a rubber material for providing a rubber composition is employed in general. Although the ring-opening polymer of cyclopentene disclosed in Non-Patent Document 1 has low dispersity for a filler like silica and carbon black and thus has an improved property at low temperature, it has a problem that the low heat generation property is poor. Further, since the ring-opening polymer of cyclopentene disclosed in Non-Patent Document 1 has low Moony viscosity, which makes the kneading at high temperature difficult, and therefore there is a problem that the processability is poor.